This application is based on German priority application 19930171.9, which is hereby incorporated in its entirety by reference.
The present invention relates to a differential amplifier, and more particularly to a selective, switchable low-noise differential amplifier in the form of an integrated circuit (IC).
The invention relates to a differential amplifier as an IC with a push-pull input, a push-pull output, two transistor amplifier stages and two cascode stages.
A similar differential amplifier is disclosed for example in xe2x80x9cHalbleiter-Schaltungstechnikxe2x80x9d [Semiconductor circuitry], page 392, Fig.4.74, Tietze/Schenk, 11th Edition, Springer-Verlag. This known differential amplifier is a cascode differential amplifier.
In mobile radio, simple amplifier stages generally comprising low-noise individual transistors have been used heretofore in the prior art, where the necessary image frequency suppression is effected by a passive filter upstream of the amplifier and by a further passive filter upstream of the mixer that is connected downstream. The required frequency selection has thus taken place outside the amplifier, which is embodied in a discrete or integrated manner.
Although previous integrated known solutions for integrated circuits were constructed in a switchable and low-noise manner, they were nonetheless constructed in an unbalanced manner. The image frequency suppression was effected by a passive filter upstream of the LNA and by a passive filter upstream of the mixer, that is to say the frequency selection took place outside the IC.
Since mobile radio equipment, and hence the modules in this equipment, too, are subject to ever greater miniaturization, efforts are made to construct the modules in an embodiment which is as compact as possible.
U.S. Pat. No. 5,352,987 shows an analog multiplexer having a differential amplifier with two transistor amplifier stages and also respective cascode stages which are coupled thereto and to which a push-pull output is connected. A multi-emitter transistor is connected to the amplifier stages and serves to effect changeover between a plurality of such differential amplifier stages.
The object of the invention, therefore, is to provide a large-scale integrated amplifier which is suitable for amplifying a specific frequency with as little noise as possible.
Accordingly, the inventor proposes improving the known differential amplifier as an IC with a push-pull input (RFin, RFXin), a push-pull output (RFout, RFXout), two transistor amplifier stages and two cascode stages such that the cascode stages can be switched by a changeover switch (DC gain switch).
An advantageous refinement of the differential amplifier according to the invention provides for there to be a selection circuit in the IC, which selection circuit restricts the gain to a specific frequency.
The differential amplifier is advantageously constructed in such a way that the two cascode stages are constructed in a balanced manner.
The selection circuit of the resonant circuit may preferably have at least one capacitor and at least one coil.
The preferred design of the at least one coil is for it to be integrated as a planar loop of at least one interconnect in the IC.
A further preferred embodiment of the cascode differential amplifier according to the invention consists in the bias supply of the transistors of at least one amplifier stage having an integrated bandgap circuit and, if appropriate, an operational amplifier connected downstream. The bandgap circuit is a term for an integrated voltage supply which provides a reference voltage that is independent of the temperature and the supply voltage.
In addition or as an alternative, the bias supply of the transistors of at least one cascode stage may also comprise a corresponding, integrated bandgap circuit and, if appropriate, an operational amplifier connected downstream.
A further refinement of the differential amplifier according to the invention consists in negative feedback, preferably parallel negative feedback, with at least one resistor and/or at least one capacitance element in each case, being provided in at least one amplifier stage, preferably in both amplifier stages.
In addition, provision may be made for conducting away the signal at the push-pull output (RFout, RFout) via a capacitance element and a resistor.
The transistors of the differential amplifier may be configured either as npn or pnp bipolar transistors or as field effect transistors (FETs).
The circuit arrangement of the integrated module according to the invention for the first time provides an integrated, selective, gain-switchable and low-noise cascode differential amplifier, which may be used for example as a preamplifier of reception signals in mobile radio equipment and assists the image frequency suppression for constant input impedance and output impedance.
The features of the invention mentioned herein can be used not only in the combination specified in each instance but also in other combinations or by themselves, without departing from the scope of the invention.